Watson is due to sit down with representatives of the Alliance for Lobbying Transparency (ALT) on Monday.

He has already met the Association of Professional Political Consultants (APPC), CIPR, PRCA and the Law Society. This week, he also met the National Council for Voluntary Organisations, to get the perspective of charity lobbyists.

It is understood he finally agreed to meet the ALT after he was publicly questioned on the micro-blogging site Twitter.

The minister is overseeing the Government’s response to the public administration se­l­ect committee report calling for greater transparency in lobbying.

The ALT is calling for a mandatory register of lobbyists and ‘enforceable ethics rules for all lobbyists’. A spokeswoman for the ALT, whose members include Greenpeace and Friends of the Earth, said: ‘It’s great that the minister is meeting with us. This shows that he’s not putting all his faith in the APPC, CIPR and PRCA to regulate the industry on their own.’

The development follows last week’s news that the APPC, CIPR and PRCA have set up a working party to explore self-regulation – but have not included the Law Society.

This week it emerged that Bell Pottinger Public Affairs had also not been consulted about the working group. Agency chairman Peter Bingle said: ‘Having an industry position without involving the market leader seems a little strange, but is sadly par for the course.’

Bell Pottinger Business & Brand is listed as a member of the PRCA, but Bell Pottinger Public Affairs is not a member of either the PRCA or the APPC.

Meanwhile, APPC chair Robbie MacDuff said membership of the working party was not set in stone: ‘My assumption is that the working party will aim to bring others around the table. It is there to begin initial discussions. It’s not a small exclusive club.’